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Two staff from the Ministry of Agriculture and Forestry's Biosecurity New Zealand were expected in Dunedin this morning, Dunedin City Council community and recreation services manager Mick Reece said.
The Maf staff planned to inspect the 119-year-old trees in the Octagon, as well as others showing signs of infection in the city, and carry out a series of more "aggressive" tests to identify the fungal disease believed to be attacking them, Mr Reece said.
Initial samples taken from the trees - as well as others in George and Princes Sts showing signs of disease - since their condition became obvious in November had been analysed by Maf staff, but preliminary results had failed to identify the disease, he said.
Maf scientists had this week agreed to escalate their response to "red light stuff", making identifying the disease a top priority for the department's laboratory staff, Mr Reece said.
"We really need to go to another level," he said.
A Wellington-based Maf spokesman confirmed staff would be travelling to Dunedin to take their own samples, rather than relying on those previously sent from Dunedin.
Identifying the disease was being given a higher priority because of the significance of the trees to the city, he said.
There was nothing in the preliminary results to indicate what was causing the trees to wilt, but it was also too soon to say if any disease attacking them could be exotic, he said.
"It's still a mystery, hence the more full tests," he said.
Results were expected in "two to three weeks", he said.
The Otago Daily Times reported on Tuesday four of the 16 plane trees in the Octagon - planted in 1891 - had failed to come into leaf as expected in November.
They were not dead, with small amounts of leaf and sap under bark, but had shown no signs of recovery despite regular rainfall and pruning, Mr Reece said.
The trees had a history of problems with anthracnose, a fungal disease considered the "common cold" of trees.
They usually recovered with pruning, Mr Reece said.
"This is different," he said.
Whatever the disease was, it had the potential to spread, he said.
The worst-case scenario was the loss of all 16 plane trees, with the council being forced to replace them with a completely different type.